Photothermographic materials have been proposed from of old. For instance, such materials are disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075, and Thermally Processed Silver Systems written by D. Klosterboer in the book entitled "Imaging Processes and Materials", 8th ed., .sctn.9th, p 279, compiled by J. Sturge, V. Walworth & A, Shepp, published by Neblette in 1989. In general a photothermographic material has a photosensitive layer containing in a binder matrix a catalytic amount of photocatalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., an organic silver salt) and, if needed, a toning agent for controlling the tone of silver. After imagewise exposure, the photothermographic material is heated at a high temperature (e.g., at least 80.degree. C.) to cause a redox reaction between silver halide or a reducible silver salt (functioning as an oxidizing agent), thereby forming a black silver image. The redox reaction is accelerated by the catalytic action of a latent image formed from the silver halide by exposure. Accordingly, the black silver image is formed in the exposed area.
The photosensitive layer of a photothermographic material has so far been formed using a coating solution containing polyvinyl butyral (PVB) as a binder in an organic solvent. However, the use of an aqueous solvent for the coating solution has been desired from the working viewpoint of preparation and application of a coating solution and the instrumental viewpoint. As a solution to such a problem, the method of coating an aqueous dispersion of styrene-butadiene copolymer for the formation of a photosensitive layer has been proposed in, e.g., JP-A-10-186567 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the protective layer made of gelatin has turned out to crack when it was provided on the photosensitive layer using the foregoing styrene-butadiene copolymer as binder and underwent heat development. This cracking problem is not produced in the case where the protective layer made of gelatin, cellulose or the like is provided on the photosensitive layer using PVB as binder.